Step-graded foil and paper insulation systems and capacitive insulation systems are generally employed on high voltage measurement and control apparatus such as current transformers for the purposes of protecting personnel from shock hazard and electrical instrumentation from equipment damage. An example of a step-graded system comprises multiple alternating conductive and dielectric layers, with the conductive layer of least potential being earth grounded. The alternate layers are used to form an effective series capacitive divider circuit between the high voltage conductors and ground potential. These alternate layers are usually made from foil and paper. The paper dielectric is usually oil impregnated and is generally used in oil-filled instrument transformers, power transformers, condenser bushings and other apparatus for high voltage electrical power systems. Some SF6 gas insulated systems use a metallized film type of capacitive insulation.
Most step graded insulation and capacitive insulation systems are designed such that the capacitance of each pair of alternate layers is equal, thus producing an equal voltage stress on the dielectric between each conductive layer when the apparatus is energized at high voltage. In designs where each layer is of equal capacitance, the total capacitance of the insulation system is equal to the layer capacitance divided by the total number of layers. A charging current through the capacitive circuit exists and is directly proportional to the product of the line voltage, the line frequency, and the total capacitance. With the line voltage and frequency relatively constant, changes in the insulation charging current are due primarily to a degradation in the insulation system. Electrical breakdown between layers results in degradation of the oil purity which leaves carbon deposits, providing a conductive path which effectively constitutes a short circuit between adjacent foil layers. The total capacitance of an insulator exhibiting such degradation increases as the effective number of layers is reduced. This increase in total capacitance will increase the charging current. Furthermore, each of the remaining layers is subjected to an increase in voltage stress. Ultimately, as additional layers break down, the residual voltage stress between the remaining layers may exceed safe operating levels, leading to the eventual, often catastrophic, failure of the entire insulation system.
Conventional high voltage measurement and control equipment which employ foil and paper step-graded or capacitive insulation offer no inherent means for monitoring the insulation charging current. Methods have been developed for monitoring the condition of the insulation apparatus. Most of them employ off-line methods. A power factor test requires that the system be energized with a test voltage and changes in the measured power factor or capacitance over time are recorded to see if there are any significant changes that would indicate a shorted layer. Partial discharge methods are effective in detecting these changes, but must be performed off-line and may not be practical in installations where interruption of service is not economical. Another method, gas-in-oil analysis, requires an oil sample to be drawn and tested to determine the presence of various gas that are generated when the apparatus overheats, usually indicative of a breakdown of the insulation. Some other prior art systems employ a measuring resistor in series with the ground loop and measure the voltage generated by the leakage current. However, direct measurement of this voltage is often misleading due to the lack of compensating networks to overcome the influence of the capacitance of the insulation and effects of electrical interference. Sensing the insulation charging current may not be satisfactorily accomplished by means of a resistive series element in the grounded electrode or by means of a ferrous magnetic core device. In either the resistive or ferrous magnetic sensing method, the capacitive nature of the insulation circuit between the high voltage conductor and ground is disturbed by a resistive or inductive sensor to the point where the magnitude of the insulation current is altered. Other methods inject a current at a lower frequency than the network and detect the resultant current flow in the effective leakage resistance and capacitance. These methods, being applied off-line, are incapable of continuously monitoring for a change in the insulation charging current while the apparatus is in operation. Further, they are often intrusive to the hermetically sealed insulation common to these types of insulation systems.
Commonly assigned U.S. Pat. No. 5,471,144 describes an on-line improvement over these common methods for monitoring the quality of electrical network insulation. In this system, a remote sensing coil produces a voltage output that is linearly proportional to the insulation charging current and a remote, self powered electronic circuit coupled to the sensor modulates a DC current control circuit proportionally to the output voltage of the sensor. An electronic control circuit provides a suitable voltage supply for the modulated current and alarm threshold detection circuits within the control circuit compare the output proportional voltage with predetermined levels. Although this system provides accurate results, installation of the monitor is such that the sensing coil may be mounted under oil in the transformer tank with the transmitter being located in a secondary box of the high voltage apparatus. This further requires an air-oil feed-through to pass wiring between the sensor coil and the transmitter. The insulation monitor becomes dedicated to the transformer and can not be used to monitor another transformer without extensive downtime and effort. Retrofitting an existing installation also requires a shutdown of the high voltage apparatus. Since the coil is mounted within the insulating oil, the unit has to be removed from the installation site, drained and dismantled before the coil can be installed. The unit has to be refilled under vacuum and the insulating oil reprocessed to remove impurities and moisture. This is time consuming and not very cost effective.
A capacitance tap is an existing electrode provided on all condenser bushings used on high voltage power transformers and circuit breakers and is also used on current transformers. The tap provides access to the insulation capacitance for off-line testing purposes and could be used for measuring voltage on-line. It would be an advantage to have an apparatus that couples directly to this tap for monitoring the insulation quality of the high voltage equipment. This will allow onsite retrofitting of existing installations without requiring dismantling and draining of the insulating oil from the equipment.